Enable sRGB on iOS, make sRGB decode support optional

tests/SRGBMipMapTest.cpp

Index: tests/SRGBMipMapTest.cpp
diff --git a/tests/SRGBMipMapTest.cpp b/tests/SRGBMipMapTest.cpp
index a875c5c71033afcba2d57daf008d1eaf63932fbf..88f08d43d73e77a214f55bfd775608ef76f63f1a 100644
--- a/tests/SRGBMipMapTest.cpp
+++ b/tests/SRGBMipMapTest.cpp
@@ -10,6 +10,7 @@
 #include "GrCaps.h"
 #include "GrContext.h"
 #include "GrRenderTargetContext.h"
+#include "gl/GrGLGpu.h"
 #include "SkCanvas.h"
 #include "SkSurface.h"
 
@@ -142,8 +143,18 @@ DEF_GPUTEST_FOR_GL_RENDERING_CONTEXTS(SRGBMipMaps, reporter, ctxInfo) {
     // 2) Draw texture to L32 surface (should generate/use linear mips)
     paint.setGammaCorrect(false);
     l32RenderTargetContext->drawRect(noClip, paint, SkMatrix::I(), rect);
-    read_and_check_pixels(reporter, l32RenderTargetContext->asTexture().get(), expectedLinear,
-                          error, "re-render as linear");
+
+    // Right now, this test only runs on GL (because Vulkan doesn't support legacy mip-mapping
+    // skbug.com/5048). On GL, we may not have sRGB decode support. In that case, rendering sRGB
+    // textures to a legacy surface produces nonsense, so this part of the test is meaningless.
+    //
+    // TODO: Once Vulkan supports legacy mip-mapping, we can promote this to GrCaps. Right now,
+    // Vulkan has most of the functionality, but not the mip-mapping part that's being tested here.
+    GrGLGpu* glGpu = static_cast<GrGLGpu*>(context->getGpu());
+    if (glGpu->glCaps().srgbDecodeDisableSupport()) {
+        read_and_check_pixels(reporter, l32RenderTargetContext->asTexture().get(), expectedLinear,
+                              error, "re-render as linear");
+    }
 
     // 3) Go back to sRGB
     paint.setGammaCorrect(true);